Using Dichotomous Keys to Identify
Arachnids and Insects
Planning A:
Objectives: • Use a dichotomous key to identify types of arachnids.
Construct a dichotomous key for the identification of some insects.
Hypothesis: none
Variables: the different pictured arachnids and insects.
Background: A dichotomous key, based on visible physical traits, can help you identify
members of a group of related organisms. Dichotomous means branching
into two parts. As you use a dichotomous key, you read pairs of descriptions
and decide which description of each pair is true about the organism you are
identifying. You still to include your own background on why insects are
the most successful arthropod class.
Planning B: Summarize procedures for both parts. Remove any directions or images that aren’t
part of your write-up.
Materials: Pen and paper. Optional: Field guide to arachnids and insects
Crab spide r Yellow Vaejovis
Part A: Using A Dichotomous Key (14 pts)
1. Before you use the dichotomous key to identify the arachnids on the attached page, review
arachnid anatomy by studying the diagrams above. In all arachnids the body is divided
into two sections: the cephalothorax and the abdomen. Find the cephalothorax and the
abdomen in each arachnid. Note how these body sections are joined. Observe whether the
abdomen is segmented or lacks segments (example, “h” is segmented). All arachnids have
six pairs of appendages: one pair of jaws, eight walking legs and two pedipalps.
2. Use the dichotomous key on the next page to identify each arachnid (a-n). Beginning with
the first pair of statements, decide which statement is true for the arachnid. Proceed to the
next pair of statements as directed by the key. When the key lists a name rather than a
number of a statement pair, then you have identified the arachnid. Be sure to record each
step of the key you used to identify the arachnid.
Dichotomous Key:
1.a) Cephalothorax or abdomen segmented.............................................................................go to 2
b) Neither cephalothorax nor abdomen segmented.......................................................... go to 9
2.a) Abdomen with tail ................................................................................................................go to 3
b) Abdomen without tail .......................................................................................................go to 5
3.a) Thick tail with stinger at tip ............................................................................................ Scorpion
b) Slender tail without stinger at tip ....................................................................................go to 4
4.a) Pincers on pedipalps; large arachnid ................................................................... Whipscorpion
b) Pincers on jaws; small arachnid ............................................................... Microwhipscorpion
5.a) First leg long and whiplike ........................................................................................ Whipspider
b) Legs about equal length ....................................................................................................go to 6
6.a) Abdomen much smaller than cephalothorax ............................................................ Sea spider
b) Abdomen as large or larger than cephalothorax........................................................... go to 7
7.a) Pedipalps longer than legs.................................................................................. Pseudoscorpion
b) Pedipalps about the same size or shorter than legs ......................................................go to 8
8.a) Legs long and stiltlike ........................................................................................ Daddy long legs
b) Legs not longer than body; pedipalps lack pincers ....................................... Wind scorpion
9.a) No narrowing where cephalothorax and abdomen join ...............................................go to 10
b) Narrowing where cephalothorax and abdomen join .................................................go to 11
10. a) Tiny and covered with spines ............................................................................................. Mite
b) Large, oval body; few spines if any ..................................................................................... Tick
11. a) Entire body and all legs covered with "hair" ............................................................... go to 12
b) Only parts of body covered with "hair" ........................................................................go to 13
12. a) Pedipalps curved................................................................................................ Jumping spider
b. Pedipalps straight ........................................................................................................ Tarantula
13. a) Large, thick jaws that are close together;
cephalothorax covered by plate .................................................................... Trapdoor spider
b) Small, thin jaws; long, hair-covered legs;
cephalothorax covered with short hairs...................................................................... Argiope
Arachnids:
Part B: Making Your Own Dichotomous Key (6 pts)
Study the drawing of the insects below. On a separate piece of paper, construct a
dichotomous key for the identification of the insects. Use the key from part A as a model.
Use your textbook to identify the major anatomical features of an insect. You cannot use
more than two choices for every key step.
Insects:
Conclusion:
1. How is “dichotomous” an appropriate word to describe the keys you used and constructed in
this laboratory?
2. Give three visible traits common to all arachnids. (3 pts)
3. State why behavioural traits, such as “preys on small insects” or “spins intricate web,” would
not be appropriate for these dichotomous keys?
4. Give 3 visible differences between arachnids and insects? (3 pts)
• Don’t forget error analysis and suggestions